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How to Calculate your Decking Area

No two decking plans are the same in terms of layout or size. For this reason we use a general method that can be applied to 99% of decking plans to calculate the total decking area.

Three Simple Steps

  • Break your plan into rectangular areas.
  • Calculate the area of each rectangular area.
  • Add the results of all rectangular areas together

The above will give a result that is called the Total Decking Area

Not So Simple !

If every decking plan was made up of simple rectangles life would be so much easier, but as decking plans get ever more complex with curves, wedges & other one off unique shapes then the basic, divide into rectangles approach becomes less & less accurate.

It is important to understand that such area calculation techniques when applied to more complex area shapes are still just a best estimate of your requirements.

The Basic Rectangle Plan

The simplest & easiest to calculate of decking plans is the single rectangle / square. we will start with this then move on to more advanced decking plans.

We will assume that the rectangular / square decking area is going to be laid at ground level with no raised sides. We will cover raised decking later as raised decking requires further areas of decking board to cover the raised sides.


Single area decking plan

The Calculation

We will assume the following measurements:

Length : 3.2 Meters (320cm)
Width : 2.2 Meters (220cm)
Total Decking Area : 3.2(L) X 2.2(W) = 7.04 M2

Identifying Our Rectangles

For none rectangular / square decking plans we need to identify the individual rectangular areas that make up our more complex shape. As a general rule it is normally best to select rectangular areas based on the longest sides of your plan, we demonstrate this in the diagrams below.

Remember that it's always best to consider your plan carefully & consider the consequences of selecting a rectangular area on the rest of your decking plan.

2 Section decking Plan
2 Section decking Plan B
3 Section decking Plan
3 Section decking Plan B

Complex Shaped Areas

Any complex shape can be contained within a 'bounding rectangle'. Calculating the area of this bounding rectangle you can calculate the maximum decking board area that you will require. Subject to the shape & some imaginative thinking on your part you can see what part of any wastage / off-cuts can be utilised in other areas of your complex design, or even make an out-door coffee table!

Given the infinite number of complex shapes, for the purpose of this text we will stick with the 'bounding rectangle' approach.

Semi Circle decking Plan
Complex Arc Decking Plan
Complex decking Plan

Tools for Composite Decking

As a general rule of thumb normal woodworking tools can be used for cutting shaping & forming composite decks.

The same safety rules that apply for using woodworking tools also apply to working with wood composites. A point to bear in mind is that the none wood component of composite can cause high speed circular saws to heat & at time stick.

Raised Decking

Raised garden decking plans add yet further complexity to the calculation of total decking area. It is not just the height & length of the raised areas. We also need to take into account what areas will be exposed, as some rectangular areas will be adjacent to such as : other rectangular decking areas, garden fences, walls of buildings etc. Such areas will not be exposed and so will not generally need to be covered with extra decking boards. Add to this that not all rectangular areas will be raised, some may be adjacent to other rectangular decking areas, but at a lower height.

You will need to calculate what sides of which areas will be exposed & to what height, subject to your decking layout & from a side view apply the simple rectangular calculation for each exposed area, adding these areas in turn to your total decking area requirements.

Decking Joists / Frame

The decking frame is the structure that holds your decking plan / design together. As with the variants of the calculation of total decking area that is based on the complexity of the decking plan. The same is also true for calculating the requirements for your decking frame / joist requirements.

What You Need to Know

  • There must be a joist at the edge of every rectangular area of decking.
  • There must be a joist every 40cm across each rectangular area to safely support the decking boards.
  • Raised decking plans will require down joists to support the upper frame.
  • Any raised deck that is higher than 30cm requires local planning permission.

Use Composite Joists

Many people use natural wood timber for their decking frames. While natural wood can be used for decking frames it can defeat the object of of using long lasting maintenance free composite decking boards when your decking frame starts to rot. In such situations the deck will need to be disassembled & replacement joists fitted. This added expense & reduced life of your deck eventually exceeds far more than any saving that thought you will have made by using sub-standard timber frames / joists.

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